Tiger, Harry, ah thanks. Ain't got a rod, got this
The thicker section goes into the back of the floating shelf. So I need the rawlplug - or something - to centre the fitting in the hole.
(It's in and the shelf is up, but it's a botch!)
Those are designed to be screwed into wood, not masonry (the thread is a tapered wood thread, not a parallel machine screw thread like the original had), and I'd hazard a guess that the holes you have in the wall are far too big for it in any case. As you have surmised, the threaded end goes into the wall. But my suspicion is that the holes are already too large for any plug you could find to work with that screw and that attempting to use this type of fixing may well not work, or work reliably enough
Assuming from the previous photo you showed that the holes have had resin injected into them
and that there are no shield ends inside the wall (as you would have with shield anchors aka a Rawlbolts), what I suggest is that, as you already have a sample of threaded stud, it should be possible to get hold of matching replacement studs (looked them up on Toolstation and they are quite reasonable - M12 x 160 cost under £9 for 10 no -0 assuming you don't have the tools to cut and clean-up threaded rod, namely a hand file, and a hacksaw or an angle grinder) and a couple of matching nuts. You'll need to extract the damaged old studs and wind in new ones (hence the two nuts - to act as a locknut).
After the studs are in the shelf redrill the holes in the wall, same diameter as the threaded rod PLUS 2mm - so if your rod is M12 you'll need a 14mm drill bit. As you are only drilling resin this can possibly be done with a HSS reduced shank blacksmiths's twist drill in an ordinary drill or with an SDS drill and 14mm SDS bit. The holes can be cleaned out using a blower, a hole brush or a vacuum cleaner and crevice tool. Do a dry run to ensure you can get all the studs in to full depth and adjust as required
Next the holes will need to have resin injected into them. It is widely
available and not overly expensive, but it does require a
special mixing nozzle (which may or may not come with the resin - ask first) and it is best applied with a
heavy duty caulking gun as the cheap DIY skeleton guns are generally not up to the task. Put a generous bead of resin inside the holes along the bottom of the hole from the deepest part to the front - pulling the gun towards you. Repeat for all holes. The put a bead along the top of the protruding threads. Finally gently push the shelf and threaded rods onto the wall and prop the mantlepiece with a broomstick to stop it drooping whilst the resin sets. This all needs to be done promptly as the resin starts to set within 5 or 6 minutes of mixing
You need a good hole for the resin to soak in and then be squished out as the rod goes in
I agree that you need a good clean hole, but resin doesn't soak into anything - it does, however, flow into pores and fissures in the masonry and sets to form a plug which is very difficult to remove
